Cellular Networks: Cells, Handoffs, Frequency Reuse & 1G-5G Evolution

TAKEAWAYS:

• Cellular networks divide coverage into hexagonal cells (macro, micro, pico) to reuse frequencies and maximize capacity, with frequency reuse factor (K) determining interference levels.
• Handoff (handover) between cells is triggered by signal strength thresholds (e.g., -85 dBm) and uses soft/hard handoff techniques to maintain call quality.
• Cellular generations (1G–5G) differ in modulation, bandwidth, latency, and use cases (e.g., 4G LTE for mobile broadband, 5G for IoT and ultra-low latency).
• Frequency reuse planning balances capacity vs. interference via cluster size (K) and co-channel interference (CCI) mitigation (e.g., sectorization, power control).
• Ncell/NTC networks in Nepal use GSM (2G) and LTE (4G) with 700 MHz/1800 MHz bands, while global operators like Verizon deploy mmWave (24 GHz) for 5G.
• Worked example: Calculate the number of channels per cell for a system with 1200 total channels and K=7 (reuse factor).

1. Cellular Network Basics: Cells and Coverage

Cellular networks divide geographic areas into cells to efficiently reuse radio frequencies. Each cell has a Base Transceiver Station (BTS) or eNodeB (4G/5G) that communicates with mobile devices.

Cell Types and Sizes

Why hexagonal cells?

• Optimal coverage: Hexagons minimize overlap and gaps when tiled.
• Frequency reuse: Adjacent cells use different frequencies to reduce interference.
• Interference modeling: The 4-cell reuse pattern (K=4) is simplest, but K=7 is common for balance.

graph TD
    A["Macrocell\n(35 km)"] -->|"Covers"| B["Rural Areas\n(Nepal hills)"]
    C["Microcell\n(2 km)"] -->|"Covers"| D["Urban Areas\n(Kathmandu)"]
    E["Picocell\n(100 m)"] -->|"Covers"| F["Indoor\n(NEPSE hall)"]
    G["Femtocell\n(10 m)"] -->|"Covers"| H["Home/Office"]

2. Frequency Reuse and Cluster Size (K)

Problem: Limited spectrum must serve many users without interference. Solution: Frequency Reuse – Assign the same frequency to distant cells.

Key Terms

• Cluster (K): Number of cells sharing the same frequency set.
  • K=1: No reuse (inefficient).
  • K=4: Simple but high interference.
  • K=7: Balanced (used in GSM).
• Co-channel Interference (CCI): Signals from same-frequency cells overlap.
• Adjacent Channel Interference (ACI): Signals from nearby frequencies leak.

Formula for Number of Channels per Cell: $$\text{Channels per cell}=\frac{\text{Total channels}}{K}$$ Worked Example (Ncell 2G Network):

• Total channels = 1200
• Reuse factor $K=7$
• Channels per cell = $\frac{1200}{7}\approx 171$

3. Handoff (Handover) Mechanisms

When a mobile device moves between cells, a handoff ensures seamless connectivity.

sequenceDiagram
    participant MS as Mobile (Ncell User)
    participant BTS1 as Tower A (1800 MHz)
    participant BTS2 as Tower B (1800 MHz)
    participant MSC as Ncell Core Network

    MS->>BTS1: Signal drops to -86 dBm (below threshold)
    BTS1->>MSC: Handoff request
    MSC->>BTS2: Allocate resources
    BTS2->>MS: Pilot signal (soft handoff)
    MS->>BTS1: Maintain link
    MS->>BTS2: Sync established
    BTS1-->>MS: Release old link (graceful)
    note right of MS: No call drop during handoff

Types of Handoff

Handoff Trigger Conditions:

• Signal strength drops below threshold (e.g., -85 dBm).
• Timing advance (delay in signal round-trip) exceeds limit.
• Traffic load balancing (e.g., offloading to less busy cells).

Mermaid Sequence for Soft Handoff:

sequenceDiagram
    participant MS as Mobile Station
    participant BTS1 as Base Station 1
    participant BTS2 as Base Station 2
    participant MSC as Mobile Switching Center

    MS->>BTS1: Signal strength < -85 dBm
    BTS1->>MSC: Request handoff to BTS2
    MSC->>BTS2: Allocate resources
    BTS2->>MS: Sync signal (pilot channel)
    MS->>BTS1: Maintain connection
    MS->>BTS2: Establish link (soft handoff)
    BTS1-->>MS: Release old link (graceful)

4. Cellular Generations (1G–5G): Evolution

Key Differences:

• Modulation: 1G (AM/FM) → 4G (OFDMA) → 5G (OFDM + mmWave).
• Spectrum: 2G (900 MHz) → 5G (sub-6 GHz + 24 GHz mmWave).
• Latency: 4G (~30 ms) → 5G (<1 ms for URLLC).

5. Real-World Applications in Nepal

1. Ncell/NTC Network Planning

• Problem: Kathmandu’s dense traffic causes call drops due to handoff failures.
• Solution: Ncell uses microcells (1800 MHz) in Thamel and picocells in malls to reduce handoff distance.
• Frequency Reuse: $K=7$ in urban areas to balance capacity and interference.

2. NEPSE Trading Hall (Low-Latency Picocells)

• Challenge: High-frequency trading requires <10 ms latency.
• Solution: Dedicated picocells (5 GHz Wi-Fi + LTE) with softer handoffs between access points.

3. Pathao Driver Routing (5G Potential)

• Current (4G): GPS updates every 500 ms → delays in traffic rerouting.
• Future (5G): <10 ms latency could enable real-time path optimization for drivers.

6. Exam Tip: How This Unit is Tested

1. Definitions & Formulas:

   • Know cluster size (K), co-channel interference (CCI), and handoff thresholds.
   • Memorize the frequency reuse formula: $\text{Channels per cell}=\frac{\text{Total channels}}{K}$.

2. Diagrams:

   • Draw a 7-cell reuse pattern (K=7) and label CCI regions.
   • Sketch a soft handoff sequence diagram (as above).

3. Worked Examples:

   • Given: Total channels = 840, $K=4$. Find: Channels per cell. Answer: $\frac{840}{4}=210$.
   • Scenario: A mobile at -80 dBm in Cell A moves to Cell B at -75 dBm. Question: Will a handoff occur? Answer: Yes (if threshold is -85 dBm).

4. Comparisons:

   • 2G vs. 4G: 2G (circuit-switched, GSM), 4G (packet-switched, LTE).
   • Hard vs. Soft Handoff: 2G uses hard; 4G/5G uses soft.

5. Real-World Links:

   • Ncell’s 4G: Uses LTE (FDD) on 1800 MHz with $K=7$.
   • NTC’s 5G Trial: Tests mmWave (24 GHz) for ultra-low latency.

Final Note: Focus on frequency reuse, handoff mechanisms, and generation differences—these are the most tested topics. Always relate answers to Ncell/NTC examples in Nepal.

In the real world

• Ncell’s 4G LTE in Kathmandu: Uses microcells (1800 MHz) with K=7 frequency reuse to handle high traffic in Thamel and Patan. The soft handoff mechanism ensures seamless switching between towers, reducing call drops during festivals like Dashain.
• NEPSE Trading Hall: Deploys picocells (sub-100 m radius) to provide low-latency (<30 ms) connectivity for high-frequency trading. The small cell size minimizes handoff delays during volatile market conditions.
• NTC’s 5G Trials: Tests mmWave (24 GHz) in Lalitpur for ultra-low latency (<10 ms) applications like remote surgery simulations, demonstrating Nepal’s push toward 5G despite limited spectrum.